Currently, molded friction elements as installed in vehicle brake systems are molded from a blended mixture of friction ingredients that are loose, bulky and dusty blends of resinous binders, reinforcing fibers, friction and wear controlling agents and inexpensive fillers. The mix is loose, bulky and dusty because the ingredients are in the form of fine powders, small and larger granules and fibers.
Powdered phenolic thermosetting resins are the binder of choice for a variety of braking applications. These heat resistant resins serve to bond the other ingredients into a solid, infusible mass when molded to final shape under the effects of heat and pressure. The reinforcing fibers are used to impart physical strength, heat stability and, to a limited extent, friction properties to the finished element. The friction and wear controlling agents are typically granules and powders of carbonaceous materials which impart and maintain the friction level and wear resistance. The inexpensive fillers are generally powdered minerals used to fill space and enhance some properties.
The formulas of the compositions used to make friction elements for specific applications are usually developed experimentally, confirmed by testing and, to a considerable extent, are held confidential. However, generally accepted volume ranges for the various friction ingredients in the compositions are as follows:
______________________________________ % Volume Friction Ingredient general preferred ______________________________________ Binders 10-35 20-30 Reinforcing Fibers 0-50 5-25 Friction & Wear Controlling Agents 20-80 35-45 Fillers 20-70 20-30 ______________________________________
As a general rule, the binder content of a friction element is kept to a minimum. No more binder resin is used than is necessary to sufficiently bond the other ingredients into a solid, infusible mass when molded under heat and pressure. The minimum binder volume required will be determined by the types of friction ingredients used and, to a large extent, by the processes used to manufacture the final friction element.
Binder content is minimized because the organic resins used may be subjected to temperatures above their decomposition points. Partial decomposition of the binder at high service temperatures results in the formation of gases at the friction interface which lowers the element's braking effectiveness. This loss in effectiveness at elevated temperatures is referred to as brake "fade". The greater the element's binder content, the greater the effects of brake fade. Also, binders traditionally comprise a large percentage of the friction element's cost which provides an additional incentive to reduce its content.
The general process for the production of friction elements is described as follows: the desired amounts of the powdered binder resin, fibers, fillers and other ingredients are dry mixed. The bulk density of the resulting dry mix, which is a loose, bulky, dusty mass of powder, is less than half that of the molded element's final density. The mix must be carefully handled to avoid settling and separation of its powdered, granular, and fibrous ingredients.
It is current industry practice to first produce from the loose, bulky, dusty dry mix, prior to molding and at room temperature, a fragile preform in the near net shape of the final friction element. The preforms are made to the near net shape of the final element because, as a minimum amount of binder is used in friction elements to avoid brake fade, the dry mix has a very low propensity to flow and uniformly fill the mold cavity when heat and pressure is applied. The low resin content also results in weak and easily broken preforms which must be specially and carefully handled when transferred to a heated mold cavity for forming into final shape. The hot, pressurized molding step causes the resinous binder to melt, flow and coat the material's fillers. The thermosetting resin then crosslinks and cures to an infusible state. After ejection from the hot mold, the element may be cured further in an oven. When cool, the friction element is machined to final size, catalogued and packaged for market.
Preforming the dry mix prior to hot press molding helps eliminate some, but not all, of the unwanted air that can otherwise be trapped within the friction element. As the thermosetting resin crosslinks and cures during the molding step, gaseous byproducts such as water vapor and ammonia are produced. These vapors, along with the trapped air in the preform, will, if retained within the molding, result in unacceptable defects in the form of blisters, voids and delaminations in the final friction element.
In an effort to avoid such defects, it is current industry practice to frequently open the hot mold cavity during molding to allow the entrapped vapors to escape. The mold cavity is then closed and pressure reapplied so the fissures and delaminations created by the escaping vapors are sealed. This repeated opening and closing of the mold cavity is termed "breathing" or "bumping" the press. Press "bumping" cycles are continued until the binder has hardened so that the described defects will not result when the element is ejected from the mold.
Press bumping must be carefully and correctly timed so the resin has enough flow and reactivity to completely seal the voids and delaminations created by the escaping vapors. These delaminations and unsealed fissures will diminish the physical integrity of the element. Also, partially sealed fissures may delaminate when the element experiences rapid frictional heating during service. This problem has led to recalls of friction elements. Thus, press breathing, while employed to reduce defects, is time consuming, lowers productivity, and also causes product quality uncertainty.
The negative factors involved with the current methods of manufacturing friction elements can be circumvented by transforming the loose, bulky, dusty mix into a dense, regular geometry that lends itself to preheating prior to mold charging. As preheating starts the binder's curing reaction before the mixture is placed in the mold much less time is required to cure the mass of material to an infusible state. Thus, preheating the mixture is highly desirable because preheating enables more rapid molding cycles. In addition, the number of required press "bumps" may be reduced or even eliminated by preheating the charge of friction ingredients.
While high frequency radio waves, or oscillating magnetic fields, may be used to preheat specific friction material compositions, hot pressurized gas preheat systems appear to be the most economical and universally available method for preheating friction ingredients of all types. The inability to uniformly preheat conventional preforms of dry mixed friction ingredients with hot gas systems results from the mix's low density and subsequent low thermoconductivity. As heat transfer from the surface to the center is poor, simple convection or forced air ovens quickly heat and cure the binder on a preform's surface while its interior remains cool. Thus, no advantage is realized by hot gas preheating materials produced by the currently practiced dry mix preform method.
Preheating with hot, pressurized gas, requires that the friction material mix be densified in some way to improve its thermoconductivity. However, the densified particles must not be manufactured in a way that reduces the binder's plasticity to a point where the material will not flow and fuse during hot press molding. Also, as the goal is to manufacture friction elements as efficiently as possible, extra manufacturing steps such as filtering, drying, pressing, grinding or classification, which add cost and complexity to the process, should be avoided. In addition, the size and shape of the densified mix must be such that open channels are formed within a material charge to allow the hot gas to freely flow throughout all parts of the charge to assure uniform preheat.
In using hot gas preheat systems, it must be noted that while very hot air would quickly heat the friction material charge prior to molding, the preheat air must not precure the binder on the surface of a densified particle before adequately heating the interior. Therefore, while the initial thermal gradient from the particle's surface to its center must be relatively low, fast efficient preheat may be obtained by minimizing that distance. However, the particles must not be so small that they are carried away by the gas stream creating dust, or pack in a way to minimize the size of the open channels or cause large channels to form through which most of the preheat air preferentially flows.
A number of methods have been proposed for densifying a mix of friction ingredients. These methods include adding liquid solvents or cements, subjecting the mix to agglomeration techniques, or by extruding or pressing the mix followed by pulverization into a granular material. However, when put into practice these densification methods have been found to be ineffective, mainly because they add costly and complicated drying, recovery, pressing, grinding or screening steps to the manufacturing process. In addition, the densified particles obtained by these methods vary in size from inches in diameter down to small grains. The large particle size distribution results in dust generation and hinders uniform preheating with hot gas because the smaller particles quickly heat while the larger particles remain much cooler.
Thus, the problem lies in how to prepare the friction material mixtures in a way to make preheating with pressurized gas viable. We find that uncured friction material compositions formed into dense, cylindrical rod-like particles satisfies the requirements needed for efficient preheating with pressurized gas. The unique shape of the rod-like particles allows the diameter of the particle to be minimized while the length allows for the formation of a homogeneous matrix of air channels within a charge of particles. These rod-like particles are made to regular and uniform diameters on the order of 3/32" to 1/4". If the densified particles are not of the same approximate size, smaller particles will quickly heat and precure while larger particles remain unaffected resulting in a nonuniform preheat which is undesirable. The length of the rod-like particle may vary as length is more critical to particle integrity than to preheat uniformity. We have found lengths of 1/8" to 3/4" to be ideal for this purpose.